1. Field of the Invention
This invention generally relates to a process for preparing surfactant-polycrystalline inorganic nanostructured materials having designed microscopic patterns using a polycrystalline inorganic substrate. More specifically, this invention relates to biomimetically assembling inorganic thin films, and to the synthesis of mesostructured film using a supramolecular assembly of surfactant molecules at interfaces to template the condensation of an inorganic silica lattice. Additionally, this invention relates to forming an ordered silicate structure within a highly confined space.
2. Related Art
Biologically produced inorganic-organic composites such as bone, teeth, diatoms, and sea shells are fabricated through highly coupled (and often concurrent) synthesis and assembly. These structures are formed through template-assisted self-assembly, in which self-assembled organic material (such as proteins, or lipids, or both) form the structural scaffolding for the deposition of inorganic material. They are hierarchically structured composites in which soft organic materials are organized on length scales of 1 to 100 nm and used as frameworks for specifically oriented and shaped inorganic crystals (that is, ceramics such as hydroxyapatite, CaCo.sub.3, SiO.sub.2, and Fe.sub.3 O.sub.4). In some cases, structurally organized organic surfaces catalytically or epitaxially induce growth of specifically oriented inorganic thin films.
Most importantly, however, nature's way of mineralization uses environmentally balanced aqueous solution chemistries at temperatures below 100.degree. C. This approach provides an attractive alternative to the processing of inorganic thin films, especially in applications where substrates cannot be exposed to high temperatures, or more generally in the pursuit of increased energy efficiency.
Potential applications for dense, polycrystalline inorganic films span a broad range of industries. These include the possibility of applying hard optical coatings to plastics in order to replace glass, abrasion-resistant coatings for plastic and metal components subject to wear, and the deposition of oriented films of iron oxide phases for use as magnetic storage media. For many of these applications, conventional ceramic processing methods, which require high temperature sintering, cannot be used because of problems with substrate degradation.
A classic and a widely studied example of a biocomposite is the nacre of abalone shell, in which thin films of organic (&lt;10 nm) and inorganic (&lt;0.5 .mu.m) phases are coupled together to produce a laminated structure with improved mechanical properties. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of this material are shown in FIG. 1 of this application. Because of this special architecture, composites such as nacre are simultaneously hard, strong, and tough. The core of the organic template is composed of a layer of .beta.-chitin layered between "silk-like" glycine-and alanine-rich proteins. The outer surfaces of the template are coated with hydrophilic acidic macromolecules rich in aspartic and glutamic acids. Recent studies suggest that these acidic macromolecules alone are responsible for control of the polymorphic form and the morphology of the CaCO.sub.3 (calcite versus aragonite) crystals, although the role of the .beta.-chitin supported matrix on the lamellar morphology of the CaCO.sub.3 layers over macroscopic dimensions still remains to be determined.
Morphological and crystallographic analyses of the aragonitic thin layers of nacre by electron microdiffraction show that c-axis-oriented aragonite platelets form a hierarchical tiling of a twin-related dense film with twin domains extending over three length scales. Superposition of the aragonite lattices on all three possible sets of twins generates a new superlattice structure, which suggests that the organic template adopts a single-crystalline psuedohexagonal structure. Although cellular activities leading to the self-assembly or the organic template remain to be understood, the presence of organized organic template is essential to the assembly of the inorganic layer.
In recent years, a number of researchers have demonstrated the viability of this approach for the preferential growth of inorganic crystals at the solid/liquid and liquid/air interfaces. Furthermore, through chemical modification of these interfaces, by adsorbing surfactants or other reactive moieties, the crystal phase, morphology, growth habit, and even chirality of heterogeneously deposited inorganics can be controlled.
Mann et al., Nature 332, 119 (1988), describes phase-specific, oriented calcite crystals grown underneath a compressed surfactant monolayer at the air/water interface. Changing surfactant type or degree of monolayer compression results in different crystal phases and orientations.
Pacific Northwest National Laboratories (PNNL), B. C. Bunker et al., Science 264, 48 (1994), describes chemically modifying solid metal, plastic, and oxide surfaces, and the selection of phase and orientation of the depositing crystalline inorganic at a variety of solid/liquid interfaces. Bunker et al. describes the use of a self-assembled monolayer (SAM) approach to coat metal and oxide substrates with surfactant monolayers of tailored hydrophilicity. This is accomplished by pretreating the substrates with a solution of functionalized surfactants, such as sulfonic acid-terminated octadecyl tricholorsilane, before precipitation of the inorganic phase. The choice of the terminating moiety on the surfactant tail determines surface charge and relative hydrophobicity of the chemisorbed surfactant monolayer. In this way, oxide and metal substrates can be modified to have the required surface properties to promote inorganic film growth.
A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 (1993); and A. Kumar, H. A. Biebuyck, G. M. Whitesides, Langmuir 10, 1498 (1994), describe a microcontact printing method by which complex, designed SAM patterns may be transferred onto substrates with an elastomeric stamp. This approach sets up lateral variations in the .gamma..sub.is -.gamma..sub.sl value along the substrate and may be used to selectively nucleate and grow inorganic phase on the functionalized regions.
B. J. Tarasevich, P. C. Rieke, J. Lin, Chem. Mater. 8, 292 (1996); and P. C. Rieke et al., Langmuir 10,619 (1994), describe the spatially resolved deposition of FeOOH mineral through an analogous SAM approach by using electron and ion beam lithography to pattern the SAM layer. This technique allows micrometer-scaled patterning of inorganic materials on a variety of substrates through confined nucleation and growth of inorganic films.
M. R. De Guire et al., SPIE Proc., in press; and R. J. Collins, H. Shin, M. R. De Guire, C. N. Sukenik, A. H. Heuer (unpublished) describe the use of photolithography to pattern the SAM layer prior to area-selective mineralization of TiO.sub.2, ZrO.sub.2, SiO.sub.2, or Y.sub.2 O.sub.3 films.
Kim et al., Nature 376,581 (1995) describes an alternative to the SAM approach of micromolding in capillaries (MIMIC) . In this process, submicrometer-scale patterning of inorganic films is achieved by placing an elastomeric stamp, containing relief features on its surface, into contact with a substrate. Contact between the elastomeric stamp and the substrate forms a network of interconnected channels that may be filled with an inorganic precursor fluid [such as poly(ethoxynethylsiloxane)] through capillary action. After the material in the fluid is cross-linked, crystallized, or deposited onto the substrate, the elastomeric stamp is removed to leave behind a patterned inorganic film with micro-structures complementary to those present in the mold.
S. Manne et al. Langmiur 10, 4409 (1994) and H. Gaub, Science 270, 1480 (1995), have shown that three-dimensional surfactant structures such as cylindrical tubules and spheres can be formed at solid/liquid interfaces. Adsorbed hemi-micellar arrangements were observed on poorly orienting amorphous substrates, such as silica, and aligned tubular structures were observed on more strongly orienting crystalline substrates such as mica and graphite. The latter substrates orient adsorbed surfactants through anisotropic attraction (either van der Waals or electrostatic) between the crystalline substrate and the surfactant molecule. The amorphous silica substrate has no preferential orientation for surfactant adsorption.
Aksay et al., Science 273, 892 (1996) describes a method for the formation of continuous mesoscopic silicate films at the interface between liquids and various substrates. The technique used the supramolecular assembly of surfactant molecules at interfaces to template the condensation of an inorganic silica lattice. In this manner, continuous mesostructured silica films can be grown on many substrates, with the corresponding porous nanostructure determined by the specifics of the substrate surfactant interaction. XRD analysis revealed epitaxial alignment of the adsorbed surfactant layer with crystalline mica and graphite substrates, and significant strain in the mesophases silica overlayer. As the films grew thicker, accumulated strain was released resulting in the growth of hierarchical structures from the ordered film. This method was used to form "nanotubules" with dimensions of .about.3 nm. Polymerization of the inorganic matrix around these tubules leads to a hexagonally packed array of surfactant channels.
The aforedescribed techniques represent advances in the selective nucleation growth of inorganic crystals with specific phase, orientation, and micropatterns. A significant advantage of the biomimetic processing methods described above is the relatively low processing temperatures involved (typically &lt;100.degree. C.) and the use of water rather than organic solvents. Both of these factors render such methods relatively environmentally benign. Although continuous films of these silicate materials can be formed, the orientation of the tubules depends primarily on the nature of the substrate-surfactant interaction and is difficult to control. Once films grow away from the ordering influence of the interface, chaotic, hierarchical structures arise. Additionally, there is no facility for organic material to adsorb onto or to become incorporated within the growing inorganic structure or to do both.
There is thus a need for the development of low-cost lithographic techniques having the ability to pattern "designed" structural features on the nanometer size scale. Such techniques are important in the manufacture of electronic, opto-electronic and magnetic devices with nanometer scaled dimensions. Technologies involving scanning electron beam, x-ray lithography and scanning proximal probe are currently under development, but the practicality of these techniques remains uncertain. Although these continuous films hold much promise for a multitude of technological applications (e.g., oriented nanowires, sensor/actuator arrays, and optoelectronic devices), a method of orienting the nanotubules into designed arrangements is clearly required for this approach to become viable as a nanolithographic tool. What is desired and has not yet been developed is a method that allows the direction of growth of these tubules to be guided to form highly aligned, designed nanostructures. It would be desirable that the method is independent of the substrate-surfactant interaction and thus allows oriented structures to be formed on any (non-conducting) substrate.